The invention relates to the field of multi-channel optical data transmission using an optical transmission device with a plurality of parallel transmitters and, accordingly, a plurality of optical transmission media, for example optical fibers, which are connected to the transmission device and can preferably be combined to form an optical fiber strip. The invention relates to an optical transmission device having a plurality of laser transmitters that can be driven individually with a driving current to output optical signals and which have a bias current applied to them.
In such an optical transmission device, which is known from International Patent Application WO 93/13576, a plurality of individual laser diodes are disposed together as a so-called laser array and can be electrically driven individually for optical data transmission. The individual laser diodes are driven by a total current that is composed of a bias current and an individual modulation current. The bias current depends on the respective individual diode threshold current and is used to define the average optical output power, i.e. the bias current is used virtually to define the operating point of the respective laser diode. The actual data transmission, i.e. the generation of optical useful power to produce optical useful signals (for example in the form of laser pulses), is effected by superimposing the modulation current (current for producing the modulation shift) on the bias current.
The known transmission device is provided with a control loop which controls the bias current for all the laser diodes together such that the useful power of the laser diodes is kept as constant as possible. In this case, all the diodes have the same bias current applied to them. However, the control loop can be used to compensate only for influences acting on all the laser diodes in the same way or which have the same magnitude and can thus be balanced out by a common controller.
The laser diodes, which are usually produced from a common starting material, are generally situated on a common support, so that the discrepancy in their characteristic data is relatively small, and external influences (e.g. temperature) have roughly the same effect on all the laser diodes.
In practice, particularly with radio-frequency data transmission, the capacity of a parallel transmission device depends, to a not inconsiderable degree, on the similarity (uniformity) of the parameters of the individual diodes. Thus, the optical output power of the individual laser diodes can vary on account of inhomogeneous power parameters (e.g. different diode threshold currents), when all the diodes have the same current level applied to them. A small variation in the optical output power of the individual diodes in a parallel transmission device is tolerated; however, the diodes must then be preselected in this regard, which generally reduces the yield of laser arrays that can be used.
Published, British Patent Application GB 2 276 493-A discloses the simultaneous production of a multiplicity of individual laser units each with a laser transmitter, a monitor diode and a lens. In this case, a multiplicity of lasers in the form of a laser array and a corresponding multiplicity of monitor diodes and lenses are in each case firstly positioned and fixed relative to one another, with high precision, on a common substrate. The assembly is then split up to form respective individual laser units having a respective single laser transmitter. The problems related to different characteristic curves for the lasers in the (initially common) array do not, therefore, arise in operational use.
It is accordingly an object of the invention to provide an optical transmission device which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which, with the power parameters of the individual laser transmitters being essentially inhomogeneous, still has a homogeneous power spectrum for the laser transmitters.
With the foregoing and other objects in view there is provided, in accordance with the invention, an optical transmission device, including a plurality of laser transmitters each to receive and be driven individually by a driving current including a bias current and each outputting optical signals, the bias current for each of the plurality of laser transmitters having an individual correction current impressed on it and the individual correction current dimensioned such that, for a common bias current the plurality of laser transmitters have a uniform average optical output power at an operating point.
The invention achieves this object with a transmission device of the type mentioned in the introduction in that each bias current has an individual correction current impressed on it, and in that the correction currents are dimensioned such that, for the same bias current, the laser transmitters have a uniform average optical output power at the operating point.
An essential aspect of the invention is the acceptance of a considerable inhomogeneity in the optical output powers of the individual laser transmitters, by each laser transmitter being operated with an individual total bias current. This total bias current can be composed of a bias current, which is the same for all the laser transmitters, and the individual correction current, which is superimposed on the bias current. According to the invention, the correction currents are set such that, at the operational operating point, the laser transmitters have roughly the same driving-current/power characteristic curve and thus have a considerably more uniform average optical output power than in the uncorrected situation.
A significant advantage of the invention, therefore, is that it is also possible to use laser transmitter configurations for optical transmission devices whose individual laser transmitters have considerable inhomogeneity in their power characteristic curves. Individual tuning by setting the correction current can tune all the laser transmitters to the same optical output power (uniform power response).
In one embodiment of the transmission device according to the invention that is preferred in practice, a correction current source associated with the respective laser transmitter supplies the respective individual correction current. The respectively associated correction current source can preferably be produced by a resistor connected in parallel with the diode element. With particular preference, however, the respective tuning can also be carried out by externally programming the respective individual drive circuit. To this end, an externally accessible memory device can be provided which stores the necessary correction current setting in the form of a correction value or setting value individually for each laser transmitter. The correction value is used to set the size of the respective correction current, which is then generated by a current source, for example.
In terms of circuitry, it is preferable for the correction current sources and a device for defining the individual correction currents to be constituent parts of a circuit for individually driving the laser transmitters.
In a preferred development of the invention, the correction current sources and a device for defining the individual correction currents are disposed on a support that also holds the laser transmitters. This embodiment is particularly advantageous, in terms of production, because the correction devices can be easily adjusted if the laser transmitter configuration is modified, and the correction currents can be set independently of the rest of the constituent parts of the transmission device.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an optical transmission device, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.